This application is a national stage of PCT/DE99/00203 filed Jan. 27, 1999 and based upon German national application 198 02 977.2 of Jan. 27, 1998 under the International Convention.
The invention relates to a method of making a monocrystalline layer on an unmatched-lattice monocrystalline substrate in which the monocrystalline layer is formed especially by deposition on the surface of the substrate. The invention further relates to a component containing one or more such layers.
Usually the production of monocrystalline films is strongly limited by the available substrate material or the quality of the film is reduced. Different crystal structures as well as different lattice parameters between substrate and layer material (lattice defect matching) limit as a rule a monocrystalline growth of layers of higher quality. An example which is especially important for microelectronic applications is the formation of silicon-germanium (SiGe) alloys on silicon (Si). When monocrystalline layers are deposited with unmatched-lattice parameters, this has the consequence that these layers initially grow under mechanical stress, i.e. the lattice structures of the layers differ from that of the original. When the deposited layer exceeds a certain degree of stress, the mechanical stress is diminished by defect formation and the lattice structure comes closer to the original. This process is termed stress relaxation and as xe2x80x9crelaxationxe2x80x9d below.
With layer thicknesses which are usually required for components, this relaxation gives rise to defects at the boundary layer between the layer formed and the substrate whereby, however, many defects can run from the boundary layer to the layer surface (so-called threading dislocations). Since usually these dislocations develop through newly grown layers, they reduce the electrical and optical properties of the layer material significantly.
For modern telecommunications, high-speed inexpensive transistors are required. Previous transistors based on silicon do not have the desired speeds whereas those formed from combination semiconductors (GaAs) can reach these speeds. However, the use of compound semiconductors is not compatible with existing wide ranging Si technology. By the use of high quality relaxation SiGe layers, high speed transistors have been developed which show wide range compatibility with Si technology.
The state of the art in the production of, for example, stress-free qualitatively high value SiGe alloy layers on Si substrates is the use of the so-called xe2x80x9cgraded layerxe2x80x9d. In this system SiGe layers are produced with a Ge concentration which increases toward the surface until the desired Ge content is reached. Since to maintain the layer quality an increase of the Ge content of only about 10 atom percent per xcexcm can be set, such layers depending upon the Ge concentration reached can be two to three micrometers thick. For the layer growth this is not satisfactory from economical or technological points of view. The layer growth of this xe2x80x9cgraded layerxe2x80x9d is described in E. A. Fitzgerald et al., Thin Solid Films, 294 (1997) 3. This process gives rise usually to high layer roughness and incomplete relaxation.
The object of the invention is to provide a method of the type described at the outset as well as a component in which the above-described drawbacks are avoided and especially formation of threading dislocations can be avoided.
The object is achieved by forming a buried defect-rich layer in the monocrystalline substrate.
It has been found that the layer which then forms on the substrate surface is relaxed and thus that the formation of threading deformations is restricted. It has been found further that in this manner the lattice parameters of the thus formed layer come closer to the original lattice structure than the originally stressed layer such that the quality of the deposited film is not diminished by the incorporation of crystal defects according to the invention. The method of the invention thus, and in an advantageous way, can achieve a surface roughness of the formed layer which is significantly reduced by comparison to conventionally made layers. It can be advantageous to form the buried layer without damaging the surface structure of the substrate as close as possible to this surface.
The formation of the defect-rich layer buried below the substrate surface can be effected by means of ion implantation, e.g. with hydrogen can be used as the implanted ion type.
The method provides for an advantageously dislocation free deposit of the layer formed upon the substrate surface with an implantation dose in the range of 1xc3x971014 cmxe2x88x922 to 1xc3x971017 cmxe2x88x922. The ion implantation can be effected selectively either prior to the deposit of the crystalline film or after the deposit of the crystalline film to form the layer.
In an advantageous variant of the method of the invention, the ion type for implantation is selected in accordance with the substrate material for film material which is used. Especially suitable are light ions or noble gas ions.
The method according to the invention is advantageously so carried out that with further implantations the defect structure is optimized especially as to its depth. In this respect the second implantation is advantageous with a second ion type to achieve the defect density or the increase in the gas bubble density. It has been found to be very advantageous for the method of the invention to complete the thermally-induced relaxation and defect reduction by an annealing treatment.
The process according to the invention is not limited to the use of silicon substrates to form a buried defect-rich layer. Rather it can be advantageous to use a substrate material selected from the group consisting of Si, Ge, GeAs, SiC, sapphire and In P.
The component according to the invention, e.g. a blue diode based upon a sapphire substrate with a monocrystalline GaN layer or a transistor, especially a modulator field-effect transistor (ModFET), has the advantage that the microelectronic or optoelectronic characteristics of the component can be optimally configured in the layers formed without the detrimental effects or damage of threading dislocations. On the one hand, the layer fabricated according to the invention can form the desired end product. It is however foreseeable that this layer formed in accordance with the invention can constitute a suitable base, for example a buffer layer, for the growth of a further layer. In this manner, it forms a seed layer for the further growth of a monocrystalline film.
The process of the invention for producing a relaxing monocrystalline layer with limited dislocation density includes in an advantageous manner the production of a buried defect-rich layer in the substrate by hydrogen implantation This light ion type of implantation allows precise defined defect formation within the substrate to the desired depths.